Back then, we would have suffered for sending harriers over the argie mainland,where they could have used their fighters to full effect without having to worry about fuel and staying low to deploy ordinance and avoid radar. Nowadays of course, we have tomohawks and euro fighters with better refueling capabilities, as well as a real base at RAF Mount Pleasant, and the argue military capability has stagnated and degraded ever since. They would certainly be in for a world of hurt today.
Had they moved their Mirages (and reserve F-86s) to the Falklands, they could have provided real air defense against the Harriers, but they weren't that committed. I think the Argies were more worried about an attack of opportunity coming from Chile.
You are correct, that today the Argentines would be in a much, much worse position (unless they destroyed the forward deployed RAF aircraft with a special forces attack). Even then, they would be in a worse position for keeping ahold of the Falklands after they were taken.